The present invention relates to a device for supporting and transporting metal sheets during processing in a punching and/or a shearing machine.
According to the known art, a punching and/or a shearing machine involves in its different embodiments a horizontal plane on which a metal sheet is placed and moved in sequence for the various processing positions, in each of which it undergoes a punching or a shearing operation. Such movement is accomplished through a clamping manipulator which is, movable along two axes that are orthogonal to each other and horizontal.
The sheet generally has large horizontal dimensions and a small thickness and the manipulator's clamps, grasping only one side of it, are thus not in a position of supporting it in the horizontal plane without the contribution of a supporting platform extended over the entire area travelled by the sheet during processing.
Conventionally the supporting platform is provided as metal structure, in general a set of sheet metal panels reinforced by ribs, on whose the upper face there is applied, to diminish friction, a large number of supporting balls. The supporting balls are balls with a diameter ranging from one to two centimeters, which roll on a layer of much smaller balls held in a hemispherical cavity whose radius is equal to the sum of the radius of the main ball and of the diameter of the small balls; the edges of the hemispherical cavity are turned back so that the small balls cannot escape.
The presence of supporting balls on the plane supporting the metal sheet being processed, distributed uniformly over the plane itself, transforms the friction of the sheet on the supporting plane from grazing to rotating, thus solving two problems, by reducing the friction force which the manipulator must overcome and not scoring the sheet's lower face, such scoring being inevitable with the grazing contact.
The reduction in friction between the sheet and the supporting plane is not so important is allowing the increase in the accelerations of the manipulator, that are far more conditions by the inertia of the manipulator and of the sheet than by the friction, but more so, rather, in the accuracy of the positioning of the sheet on the plane. In face, in the step wherein the manipulator decelerates toward the point where the sheet is to be brought to a halt to be punched or sheared, the manipulator itself is deformed in the direction of motion of the inertia forces, which are larger than those of friction. Immediately after being brought to a halt, and before the sheet is punched or sheared, the manipulator must go back to its undeformed shape. It is precisely at this instant that the friction between the sheet and the supporting plane has great importance, because it opposes the return of the manipulator to the undeformed shape and generates inaccuracy in the position of punching or shearing on the sheet.
From this viewpoint the balls have not solved the problem completely, because they still generate some friction, friction that is sufficient to maintain the deformation of the manipulator, which tends to be increasingly lighter and thus less rigid.
The tendency towards increasing the accelerations of the manipulator in modern punching and shearing machines has highlighted another weak point of the supporting balls, which is that between the sheet and the balls a slipping action also takes place in addition to the rolling action and that as a consequence the sheet is scored along its lower face, because the ball's inertia is such as not to be able to accelerate with the sheet due to the effect of the friction's tangential force, which is small, because the sheet's portion bearing on a single ball is small.
If an attempt is made to eliminate this drawback, by reducing the inertia of the supporting balls by decreasing their diameter, the distance between one sphere and the next must perforce also be reduced, to prevent thinner sheets, when bent, from striking the balls with their edges at too high an angle of incidence, and thus, in addition to raising the cost of the balls unit, reducing the weight of the sheet on each ball and the capacity of accelerating its rotation to avoid scoring.
Thus the two problems, to solve which the supporting balls have been adopted on punching and shearing machines, heretofore have had only a partial solution, which becomes all the more unsatisfactory the greater becomes the need to increase the productivity of these machines.
On the other hand, the presence of the supporting balls on the supporting plane generates or complicates other problems such as the interference between the manipulator's clamps and the supporting plane and the evacuation of the punched or sheared sheets.
The lower jaws of the clamps clearly take up a certain space under the sheet's lower face, which rests on the balls. It is thus necessary either to prevent the interference between clamps and balls or to avoid its harmful effects.
Proposed possible solutions to these problems involve the use of flexible clamps or sprung balls, which do not avoid interference but do prevent breakages, or of cam devices which lower the balls when the clamps arrive, thus avoiding interference.
All these solutions have a financial cost that is not negligible and become increasingly critical at higher accelerations and speeds of the manipulator, made possible by the development of electric motors, of the elements for the transmission of motion and of the electronic controls of motion.
In turn, the automatic displacement of the punched or sheared sheets away from the plane on which they have been handled by the manipulator during processing cannot be carried out by the manipulator itself, because the manipulator does not have a sufficient stroke, and requires an additional device.
The ideal solution would be the concordant rotation of all of the balls, upon which the sheet would travel towards the outer part of the supporting plane, but this solution is practically impossible. Another solution would be the sub-division of the supporting plane into a plurality of segments connected together like plates of a plate-type conveyor. The latter, however, would be heavy, slow and not very effective in handling precisely because of the balls' low friction coefficient, which would have difficulty in dragging the sheet along.
Devices have therefore been adopted that are external to the supporting plane, such as handling clamps or batteries of suction cups on a movable carriage, which complicate the machine and make it more expensive.
It is therefore necessary to conclude that the supporting balls do not embody in themselves the solution of the two other accessory problems, i.e., the interference with the claps and the evacuation of the sheets.
Lastly it can be seen that the movement of the sheet on the supporting balls, also due to the non-planarity of the sheet and of the supporting plane, generates noise, because it takes place with a succession of small impacts between bodies connected to resonant structures.